1. Field of the Invention
This invention relates to the testing of underground formations or reservoirs. More particularly, this invention relates to an apparatus and a method for determining properties of formation fluids by interpreting formation fluid pressure and temperature measurements.
2. Description of the Related Art
When a wellbore is drilled, fluids from the drilling process, called filtrate, may be forced into the pore spaces of some of the earth formations, changing their fluid content and therefore their fluid properties. The process of the filtrate being forced into the pore spaces is generally referred to as “invasion”. The formation fluids may be hydrocarbon liquids and gases, and aqueous liquids, including brine. Well logging operations, either by wireline or while drilling, are used to determine properties of the formation fluids, in order to determine the potential hydrocarbon content and the locations of formation water and gas interfaces. Many of the logging systems detect the formation properties relatively close to the borehole such that the invaded zone affects the measurements. For example, when interpreting deep-reading electric well logs, it is important to know the true formation brine resistivity, which is a function of the brine salinity. When drilling with fresh water based drilling mud, the filtrate in the invaded zone can alter the resistivity of the filtrate contaminated brine in the invaded zone and substantially bias the resistivity reading. In addition, some of the highly saline brines encountered have resistivity readings that are beyond the range of presently available sensors.
Likewise, when attempting to determine gas properties in a reservoir, the filtrate in the invasion zone, affects the readings of the gas property measurements. The filtrate may also plug and contaminate sensors designed for such gas measurements. For example, it is known in the art to take a sample of the gas using a tool such as a formation tester and to pass the gas by a sensor in a flow passage in the tool for analysis. The presence of the filtrate from the invasion zone may clog such a device and/or bias the readings of such an analytical device.
A formation pressure test may be taken at multiple locations along the borehole as a formation test tool is conveyed by wireline, or in a drill string, downward through a borehole. The difference between formation pressures at two locations divided by the vertical distance between the locations produces the average pressure gradient over the interval between the test locations. The pressure gradient may be used to determine fluid density in-situ and the interface or contact points between gas, oil and water when these fluids are present in a single reservoir.
Thus there is a demonstrated need for a system and method for determining formation fluid properties that are not substantially biased by the presence of a filtrate invasion zone proximate the borehole.